Fast and reliable fault type determination of the fault in electric power lines is of great importance both for power companies dealing with electric energy distribution and for end users of electric energy. Quick and exact fault type selection affects the quality of power system protection. A means for fault phase selection and fault type determination is usually a part of a digital protection relay located in power stations or substations. Depending on the fault type, different current and voltage fault loops are distinguished and processed in the protection relay. Therefore the proper fault type selection influences the final operation of the protection relay and an error in the fault type identification may lead to mal-operation of the protection relay.
An example of a protection relay that utilizes a faulted phase selection is described in U.S. Pat. No. 4,409,636. In this solution the delta based phase selector has been known not to reliably detect three-phase faults. It calculates the percentage of the largest faulted value, and then subtracts it from the phases. If the result is a positive value the phase is detected faulted. If negative, the phase is not faulted. Often, in a three-phase fault, one of the faulted phases does not reach this level, and the phase remains undetected. Another example of a method and a device for phase selection for single-phase tripping of high-impedance ground faults is described in a U.S. Pat. No. 5,390,067. The U.S. Pat. No. 5,390,067 is limited to the selection of a single ground fault. Application of the method is limited to the networks with the earthed neutral point.
The mentioned disadvantages are overcome by the method, the device and the computer program as defined in claims 1, 2 and 3 respectively.
From JP patent application JP 2001192428 (publication No. JP2003009381) there is known a method for faulty phase selection in multi terminal system. In this method a phase selector is used, which comprises a first operating means for calculating the differential current of each phase of each transmission line; a second operating means for calculating the line differential current of a faulty line, based on the differential current of each phase obtain from the first operating means and faulty line information, a third operating means for determining the ratio of a minimum value to a maximum value obtained from the second operating means; a fourth operating means for making decision that a single phase fault has occurred, if ratio obtained from the third operating means is smaller than a specified value; a fifth operating means for determining the ratio of a differential current in the phase not constituting a maximum value to a smaller differential current, with regard to differential currents of two phases constituting the maximum value of line differential current; a sixth operating means for making a decision that a two-phase fault has occurred if a ratio obtained from the fifth operating means is smaller than a specified value; and seventh operating means for deciding that a three-phase fault has occurred, when the ratio obtained from the fifth operating means is not smaller than the specified value. Generally, there are problems with selection of a three-phase fault because one of the faulted phases does not reach a setting level. Moreover, the application of the method is limited to the networks with the earthed neutral point.
A process for identifying the type of fault detected by a protection relay is known from patent description U.S. Pat. No. 5,783,946. This process includes measuring pre-fault and post-fault samples of current waveforms on the phase-A, phase-B and phase-C conductors of a transmission line. Next increment currents DELIA1, DELIB1 and DELIC1 are calculated, whereDELIA1=(Iar−Iapr)2−(Iai−Iapi)2,DELIB1=(Ibr−Ibpr)2−(Ibi−Ibpi)2,DELIC1=(Icr−Icpr)2−(Ici−Icpi)2.
The increment currents are then employed to identify a two-phase to ground fault. In calculating the increment currents, Iar represents a post-fault value of the real part of the phase-A current, Iapr represents a pre-fault value of the real part of of the phase-A current, Iai represents a post-fault value of the imaginary part of the phase-A current and Iapi represents a pre-fault value of the real part of the phase-A current. The phase-B and phase-C value are denoted accordingly. The fault type is estimated by using crisp logical operators—greater than, less than.